Rolling plants to produce thin sheet are well known and widely used in the state of the art. There is a problem in current rolling plants, including reversing rolling plants, with respect to coordinating the rolling speed of the workpiece through the rolling stands with the rate of forming castings from continuous casters from which slabs, transfer bars and strips are formed. The coordination of the casting and the rolling should be such both to minimize the delay, and, simultaneously, the length of the passline, between the caster and the rolling stands, as well as to provide a way of controlling the casting and rolling, such that, when it is necessary to change rolls in the rolling stands, continuous casting is not interrupted.
To understand the foregoing problem, it will be helpful to refer to FIGS. 1 and 2 which schematically show a portion of a conventional rolling plant, generally designated 10. The rolling plant 10 includes a continuous caster 11 for forming a casting 20 having a thickness h.sub.a and a width W.sub.a. The casting 20 typically has a thickness h.sub.a of approximately 50 mm., and a width W.sub.a of approximately 1,250 mm. A shear 12 is provided downstream of the caster 11 for shearing the casting 20 into a series of slabs 20a, 20b having a predetermined length.
The slabs 20a, 20b are fed, sequentially, one at a time, into a temperature equalization tunnel furnace 13. The tunnel furnace 13 includes a series of rollers 13a for guiding the slabs 20a, 20b through the tunnel furnace 13. The slabs 20a, 20b exit the tunnel furnace 13 and enter into a series of four-high rolling stands 17, in the form of first, second, third and fourth rolling stands 18a, 18b, 18c and 18d, respectively. The series of rolling stands 17 and the rollers 13a in the tunnel furnace 13 move the slabs 20a, 20b through the plant 10 at a velocity equal to or greater than the casting velocity of the caster 11. The difference in the rate of formation, and therefore, the velocity of the casting 20 produced by the caster 11, and the velocity of the slab 20a in the furnace 13, creates a distance L.sub.a between the slab 20a in the furnace 13 and a slab 20b entering the rolling stands 17.
The conventional rolling plant is problematic in that in order to permit a roll change for one or more of the first, second, third or fourth rolling stands 18a, 18b, 18c, 18d, without interrupting or delaying the operation of the continuous caster 11, the length of the furnace 13 must be selected to provide a sufficient distance L.sub.a between the tail end of the last slab rolled prior to the roll change and the head end of the first slab rolled after the roll change to allow time to change the roll. In most circumstances, the distance L.sub.a can be calculated using the equation: EQU L.sub.a =V.sub.a*t
where
V.sub.a =the caster speed during roll change, which corresponds to the thickness h.sub.a and width W.sub.a of the slab. PA1 t=the time necessary to change the roll.
For example, where h.sub.a =50 mm., W.sub.a =1,250 mm., V.sub.a =5.5 m/min. and t=15 min., the distance L.sub.a is equal to 82.5 m. Thus, the rolling plant 10 is problematic in that it requires a relatively large amount of floor space for a tunnel furnace 13 of sufficient length to allow for roll changes without interrupting the continuous casting. This increases the overall cost of the rolling plant 10. Hence, a need has arisen for a rolling plant which has a relatively short tunnel furnace but is capable of conducting roll changes without interrupting or delaying the operation of the continuous caster.
The present invention provides solutions to these problems by the use of continuous casters capable of casting slabs of greater cross-sectional area and smaller cross-sectional area. The casting time for the desired thickness is coordinated with the speed of the slab to provide for a minimum length of a passline while allowing the necessary time to change the rolls of the rolling stand, when appropriate. By the techniques of the present invention, advantages of minimal limitations on steel grades, better surface quality and the ability to roll discrete plate or strip products are provided, among others.
The present inventors have studied, tested, and created and developed this invention to overcome the shortcomings of the state of the art and to achieve further advantages which will be apparent after reviewing the foregoing and following specification.